It is well-known that the use of multiple antennas at the transmitter and/or the receiver can significantly boost the performance of a wireless system. Such antenna configurations have the potential of both improving data rates and increasing coverage.
Precoding [1, 2] is a popular multi-antenna technique for improving the performance of a multi-antenna system by transforming the information carrying transmit vector so that it better fits the channel conditions. This can be done based on instantaneous channel information or long term channel information or some combination thereof. Often, precoding is implemented as performing a (linear) transformation on the information carrying vector prior to transmission. Such transformation is usually represented by a matrix. Precoding is an integral part of Long Term Evolution (LTE) as well as Wideband Code Division Multiple Access (WCDMA) systems. Precoding is also referred to as closed loop transmit diversity coding.
Precoding can be used in conjunction with any antenna configuration, resulting in correlated, uncorrelated or a combination thereof, radio channels.
Of particular interest for the present invention are antenna configurations where radio channels are correlated or a combination of both correlated and uncorrelated radio channels. FIGS. 1A and 1B illustrate two different examples of antenna configurations. FIG. 1A illustrates a simple example of a linear array of M closely spaced antenna columns having identical polarization. The column spacing may for example be in the order of 0.5 wave lengths typically resulting in radio channels having high correlation. FIG. 1B illustrates an example in the form of a linear array of N dual polarized antenna columns, where the spacing may be in the order of 1.0 wave lengths or less depending on realization, typically resulting in correlated radio channels within a polarization and uncorrelated channels between polarizations.
When antenna elements are closely spaced, and thus the radio channels typically are correlated, a beam (typically but not always narrow) is generated when the precoding vector or matrix from the codebook is applied. The vectors in the codebook in LTE, for example, are designed for, or at least best suited for, a scenario where the radio chains are coherent. This means that if the coherency is not sufficient performance will be degraded. Coherency generally describes correlation properties between physical quantities of waves (including radio and microwaves), and may be expressed in terms of for example amplitude-, time- and phase-relations. Although time and phase are interrelated, the requirements on time coherency and phase coherency are usually on completely different levels. Although there may be time coherency (signal bandwidth related) in a system, the phase (carrier frequency related) may still be more or less random.
To detect and/or ensure sufficient phase coherency some type of detection/calibration is normally required. Calibration is an area of large interest, and has been so for many years, and a considerable amount of patents exists. In a typical implementation for detection/calibration in a radio base station 10 having an antenna system 12, there normally exists a special detection/calibration branch/network 14 with an associated transceiver 15, and signals are then coupled to/from the detection/calibration branch 14 and the ordinary radio chains 16, 18 of the radio base station, as schematically illustrated in FIG. 2. For detection/calibration in the receiver direction, a calibration signal from the transceiver 15 is inserted via the detection/calibration branch 14 into all radio chains, often in or near the antennas, in order to determine how the radio chains are related in phase and/or amplitude. For detection/calibration in the transmitter direction, the detection/calibration branch 14 operates as a probe, in or near the antennas, for measuring transmitted signals in a systematic manner in order to determine how the radio chains are related in phase and/or amplitude. Suitable calibration or compensation can then be performed on the basis of the determined phase and/or amplitude relation.
Major drawbacks or problems with conventional detection/calibration implementations include the need for an extra feeder, the need for a special radio chain for transmission/reception of detection/calibration signals, and also that detection/calibration may require that normal transmission/reception is interrupted.
Reference [3] discloses an implementation not using a special detection/calibration signal, but rather based on observing the normal data to be transmitted. In this way it is not necessary to interrupt the normal operation. However, there is still a need for a special detection/calibration branch.
Reference [4] generally relates to a radio device having a number of transmission equipments, and especially to a technique for deriving a relative time delay of the transmission equipments based on at least two sub-band transmit weight vectors.